It says in the MySQL manual that you can store partial dates
http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html#function_date-format
Ranges for the month and day specifiers begin with zero due to the
fact that MySQL permits the storing of incomplete dates such as
'2014-00-00'.
This means that to store the year and month only, you can use a DATE column and put 00 instead of the day. e.g 2013-12-00.
Related: Store incomplete date in MySQL date field
